HORIZONTAL CUKVES

41

In this example C = OB; C is the length of the axis of the particular
lemniscate.

If the theodolite is set up at point 0 the various polar ray lengths
for different angles will give the points on the curve between 0 and C:
this procedure is repeated at D and curve DO defined from the same
dimensions.

For large curves with polar rays of 200 ft. or more, tacheometry
may be employed with advantage; alternatively, the curve may be
set out by offsets from the tangents.

FIQ. 22. — BERNOULLI'S LEMNISCATE FOB ROAD CURVES.

SELECTION OF TRANSITIONS
The question of the selection of a spiral or lemniscate transition
curve has been dealt with in some detail in publications by Prof.
Boyal-Dawson and Sir. H. CriswelL As previously mentioned, there
is little difference between the two curves> especially within a limited
range of deflections, and, like Prof. Royal-Dawson, the Author prefers
the lemniscate, which he has employed with advantage on several
occasions ; also for the simple parabolic curve, small deflections are
similar to the other two,
The " Unit Chord " and " Speed Chord " System,
A brief description of this method of setting out the lemniscate
curve, devised by Prof, Eoyal Dawson, is of interest. He takes a
Unit of Measurement as the Polar Bay whose deflection is sixteen
minutes,